Bone grafts have become an important and accepted means for treating bone fractures and defects. In the United States alone, approximately half a million bone grafting procedures are performed annually, directed to a diverse array of medical interventions for complications such as fractures involving bone loss, injuries or other conditions necessitating immobilization by fusion (such as for the spine or joints), and other bone defects that may be present due to trauma, infection, or disease. Bone grafting involves the surgical transplantation of pieces of bone within the body, and generally is effectuated through the use of graft material acquired from a human source. This is primarily due to the limited applicability of xenografts, transplants from another species.
Orthopedic autografts or autogenous grafts involve source bone acquired from the same individual that will receive the transplantation. Thus, this type of transplant moves bony material from one location in a body to another location in the same body, and has the advantage of producing minimal immunological complications. It is not always possible or even desirable to use an autograft. The acquisition of bone material from the body of a patient typically requires a separate operation from the implantation procedure. Furthermore, the removal of material, oftentimes involving the use of healthy material from the pelvic area or ribs, has the tendency to result in additional patient discomfort during rehabilitation, particularly at the location of the material removal. Grafts formed from synthetic material have also been developed, but the difficulty in mimicking the properties of bone limits the efficacy of these implants.
As a result of the challenges posed by autografts and synthetic grafts, many orthopedic procedures alternatively involve the use of allografts, which are bone grafts from other human sources (normally cadavers). The bone grafts, for example, are placed in a host bone and serve as the substructure for supporting new bone tissue growth from the host bone. The grafts are sculpted to assume a shape that is appropriate for insertion at the fracture or defect area, and often require fixation to that area as by screws or pins. Due to the availability of allograft source material, and the widespread acceptance of this material in the medical community, the use of allograft tissues is certain to expand in the field of musculoskeletal surgery.
With respect to the overall structure of a given bone, the mechanical properties vary throughout the bone. For example, a long bone (leg bone) such as the femur has both compact bone and spongy bone. Cortical bone, the compact and dense bone that surrounds the marrow cavity, is generally solid and thus carries the majority of the load in major bones. Cancellous bone, the spongy inner bone, is generally porous and ductile, and when compared to cortical bone is only about one-third to one-quarter as dense, one-tenth to one-twentieth as stiff, but five times as ductile. While cancellous bone has a tensile strength of about 10-20 MPa and a density of about 0.7, cortical bone has a tensile strength of about 100-200 MPa and a density of about 2. Additionally, the strain to failure of cancellous bone is about 5-7%, while cortical bone can only withstand 1-3% strain before failure. It should also be noted that these mechanical characteristics may degrade as a result of numerous factors such as any chemical treatment applied to the bone material, and the manner of storage after removal but prior to implantation (i.e. drying of the bone). In addition, bones have a grain direction similar to the grain found in wood, and thus the strength of the bone varies depending on the orientation of the grain.
Notably, implants of cancellous bone incorporate more readily with the surrounding host bone, due to the superior osteoconductive nature of cancellous bone as compared to cortical bone. Furthermore, cancellous bone from different regions of the body is known to have a range of porosities. For example, cancellous bone in the iliac crest has a different porosity from cancellous bone in a femoral head. Thus, the design of an implant using cancellous bone may be tailored to specifically incorporate material of a desired porosity.
Demineralization of cortical, cancellous, and corticocancellous bone of autograft, allograft, and xenograft types is known. In one form, bone powder or chips are chemically processed using an acid such as hydrochloric acid, chelating agents, electrolysis or other treatments. The demineralization treatment removes the minerals contained in the natural bone, leaving collagen fibers with bone growth factors including bone morphogenic protein (BMP).
The use of expandable materials as a prosthetic element is disclosed in U.S. Pat. No. 5,545,222 to Bonutti. Materials disclosed which expand when they come in contact with water or other fluids include PEEK (polyether-etherketone), a desiccated biodegradable material, or a desiccated allograft. As an example, a tendon can be compressed in a desiccated state, and as it imbibes water it expands and creates a firmer lock or tighter fit in the host site.
A shaped, swollen demineralized bone and its use in bone repair is disclosed in U.S. Pat. No. 5,298,254 to Prewett et al. In general, cortical allogeneic bone tissue is preferred as the source of bone. Demineralized bone is contacted with a biocompatible swelling agent for a period of time sufficient to cause swelling of the piece.
A flexible implant using partially demineralized bone is disclosed in U.S. Pat. No. 6,206,923 to Boyd et al. The bone implant has a first substantially rigid portion and a second substantially rigid portion which are joined by an intermediate portion that has been at least partially demineralized to create an area of flexibility in the bone implant. The pair of rigid bone portions cooperate to provide support for spacing between adjacent vertebra.
Demineralized bone has been disclosed for use as artificial ligaments in U.S. Pat. No. 5,092,887 to Gendler. Completely or partially demineralized cortical bone is sliced in strips and rods of approximately 0.1-1.5 centimeters wide and 0.1-1.5 centimeters thick with compliant elasticity and longitudinal strength similar to natural ligaments and tendons. The strips or rods are used as artificial ligaments for in vivo replacement, repair and augmentation of damaged ligaments, tendons or other fibrous tissue that permanently connects first and second body members such as the femur and tibia. Disclosure of a segmentally demineralized bone implant is found in U.S. Pat. No. 6,090,998 to Grooms et al. The implant comprises a first mineralized portion or segment, and a second, flexible, demineralized portion or segment that are produced by machining a piece of cortical bone.
A textured, demineralized, and unitary mammalian bone section for providing a rigid, foraminous, collagen scaffold for allogenic skeletal reconstruction is disclosed in U.S. Pat. No. 5,112,354 to Sires. Texturing or pore formation is carried out prior to demineralization to permit completeness of demineralization and additionally promote osteoinduction due to the increased surface area. Pores of between 200 μm and 2000 μm are created with a laser. The depth of the holes in the bone may be varied.
Also disclosed in U.S. Pat. No. 5,899,939 to Boyce et al. is a bone-derived implant for load-supporting applications. The implant is formed of one or more layers of fully mineralized or partially demineralized cortical bone and, optionally, one or more layers of some other material such as fully demineralized bone or mineral substances such as hydroxyapatite. The layers constituting the implant are assembled into a unitary structure to provide an implant with load-supporting properties. Superimposed layers are assembled into a unitary structure such as with biologically compatible adhesives.
U.S. Pat. No. 5,556,430 discloses flexible membranes produced from organic bone matrix for skeletal repair and reconstruction. Completely or partially demineralized organic bone is sliced into thin sheets. The bone may be perforated prior to demineralization, to increase the osteoinductivity of the final bone product. Similarly, U.S. Pat. No. 5,298,254 to Prewett et al. discloses demineralized bone sliced into a thin sheet which can be used to patch an injury.
A cortical bone interference screw is disclosed in U.S. Pat. No. 6,045,554 to Grooms et al. The interference screw has a cortical surface into which a self-tapping thread is machined.
In addition, U.S. Pat. No. 5,053,049 to Campbell discloses the use of milling, grinding, and pulverizing to produce pulverized bone with the desired particle size. The pulverized bone can then be combined with any suitable biologically compatible or inert carrier substance, which should have a consistency that imparts the desired flexible texture to the pulverized bone/carrier suspension, or should solidify to the desired consistency after molding or casting.
Despite these developments, there exists a need for implants formed from partially or fully demineralized cancellous bone. Furthermore, there exists a need for implants formed of bone that have been selectively masked during demineralization so that portions of the bone are at least partially demineralized while other portions are substantially remain in the mineralized state.